1. Field of the Invention
The present disclosure relates to liquid crystal display technology, and more particularly to a backlight driving circuit and a liquid crystal display (LCD).
2. Discussion of the Related Art
With the technology revolution, backlight technology of LCDs has been developed. Typical LCDs adopt cold cathode fluorescent lamps (CCFL) as the backlight sources. However, as the CCFL backlight is characterized by attributes including low color reduction ability, low lighting efficiency, high discharging voltage, bad discharging characteristics in low temperature, and also, the CCFL needs a long time to achieve a stable gray scale, LED backlight source is a newly developed technology.
Generally, backlight driving circuits are required to provide a driving voltage for LED strings. FIG. 1 is a schematic view of one conventional backlight driving circuit. As shown in FIG. 1, the backlight driving circuit includes a boost circuit 110, a LED string 120 and a constant-current driving chip 130.
The boost circuit 110 is controlled by the constant-current driving chip 130 to boost the input voltage Vin so as to drive the LED string 120. The input voltage Vin is concurrently input to the constant-current driving chip 130 such that the constant-current driving chip 130 can normally operate. The constant-current driving chip 130 receives external pulse width modulation (PWM) optical signals to control the current passing through the LED string 120 such that the LED string 120 can normally operate.
Specifically, the constant-current driving chip 130 includes a control module 131 and an operational amplifier 132. The control module 131 receives enable signals ENA such that the constant-current driving chip 130 begins its operation. The control module 131 outputs the driving signals to the MOS transistor Q1 of the boost circuit 110. When the MOS transistor Q1 is turn on, the inductor L stores energy. When the MOS transistor Q1 is turn off, the inductor L releases energy. In this way, the LED string 120 is provided with the voltage for emitting lights.
The positive input end of the operational amplifier 132 receives a constant voltage V1. The negative input end of the operational amplifier 132 feedbacks the voltage at the two ends of the resistor RT. The output end of the operational amplifier 132 couples with the gate of the MOS transistor Q2. The operational amplifier 132 compares the constant voltage V1 with the voltage at two ends of the resistor RT and then outputs control signals to adjust the voltage difference between the gate and the source of the MOS transistor Q2. As such, the current passing through the LED string 120 is controlled. The duty-cycle ratio of the current passing through the LED string 120 is determined by the duty-cycle ratio of the PWM optical signals. When the PWM optical signals are at high level, the operational amplifier 132 is capable of controlling the MOS transistor Q2. When the PWM optical signals are at low level, the operational amplifier 132 is unable to control the MOS transistor Q2. The MOS transistor Q2 is in an off state and there is no current passing through the LED string 120.
However, parasitic capacitance exists between the gate and the source of the MOS transistor Q2. When the external voltage is applied to the gate and the source of the MOS transistor Q2, the parasitic capacitance C is firstly charged. After the parasitic capacitance C is fully charged, the MOS transistor Q2 is turn on if the external voltage still exists. When the frequency of the PWM optical signals is fixed, the duration of the adjusting signals outputted from the operational amplifier 132 to the MOS transistor Q2 is short. In this way, the charging time of the parasitic capacitance C is short, which may results in that the MOS transistor Q2 cannot be fully turn on and the current passing through the LED string 120 cannot reach a predetermined level. It is to be noted that the predetermined level relates to the current capable of driving the LED string 120 to emit lights normally. Especially, if the duty-cycle ratio of the PWM optical signals is too small, the MOS transistor Q2 may not be in be turn on in time. As such, the constant-current driving chip 130 may erroneously determine that the LED string 120 is in the open-circuit state, which affects normal operations of the backlight driving circuit.